Comet 3I/ATLAS is only the third interstellar visitor to our solar system observed by astronomers. Now observations with the radio telescopes of the ALMA array in Chile provide new clues to its origin. They show that the interstellar comet’s water ice contains an unusually high proportion of the heavy hydrogen isotope deuterium. The value measured using radio spectra is around 40 times higher in 3I/ATLAS than in the terrestrial oceans and 30 times higher than in comets in the solar system. This suggests that the extrasolar comet must have formed in a colder cosmic environment than our sun and the solar system – and probably at the cold outer edge of its protoplanetary disk.
The interstellar comet 3I/ATLAS, discovered in July 2025, raced through our planetary system on a hyperbolic orbit and passed the point closest to the sun in this flight on October 29, 2025. The icy chunk, possibly a good five kilometers in size, is now on its way out of the solar system again. But its passage provided astronomers worldwide with a unique opportunity to study an interstellar object. Initial observations suggested that 3I/ATLAS is probably older than our solar system. Its ice-rich surface and outgassing also contain more carbon dioxide than the comets of our solar system. In spring 2026, data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile showed that the interstellar comet also outgassed unusual amounts of methanol.
40 times more deuterium than in our oceans
Now astronomers have discovered another special feature of the interstellar visitor. For their study, Luis Salazar Manzano from the University of Michigan and his colleagues evaluated observations from the ALMA array from November 4, 2025 – around six days after the perihelion passage of 3I/ATLAS. The focus was on spectral lines in the radio range that were generated by water molecules in the comet’s coma. The resolution of the coupled ALMA telescopes is high enough to use these lines to show the proportions of normal hydrogen (H) and the heavy hydrogen isotope deuterium (D) in these spectra. “The ratio of deuterium to hydrogen provides us with a strong chemical indicator of where and under what physical conditions this water was once formed and to what processes it was subsequently exposed,” explain Salazar Manzano and his colleagues. “The process that accumulates deuterium in water molecules reacts sensitively to local conditions such as temperatures, density and ionization rate.”
The analyzes revealed values that are far beyond those usual in the solar system. “The amount of deuterium relative to normal hydrogen in 3I/ATLAS is higher than anything we have seen before in planetary systems,” reports Salazar Manzano. With a D/H ratio of 6.6 thousandths, the deuterium content of the interstellar comet exceeds that in the terrestrial oceans by around 40 times. Compared to comets in the solar system, 3I/ATLAS is around 30 times higher and the interstellar medium does not have such a strong deuterium enrichment, as the astronomers explain. This makes the interstellar comet a truly exotic species in our cosmic neighborhood.
Cold educational environment
The exciting thing about it: The high deuterium content of 3I/ATLAS provides some clues as to the environment and under what conditions this interstellar object once formed. Accordingly, the comet’s home star must have formed in a different environment than our sun. “It is likely that the conditions were much colder, as this promotes deuterium accumulation in the water,” the astronomers explain. Another factor was the protoplanetary disk in which the comet formed. “The increased D/H ratio may indicate that 3I/ATLAS formed beyond the CO2 snow line of its home system,” the team writes. The CO2 snow line indicates the distance from the central star at which it is so cold that carbon dioxide freezes out. This could be consistent with the fact that the interstellar comet also contains a particularly large amount of CO2 ice and outgasses it. “Such formation at the outer edge of its protoplanetary disk could also explain why 3I/ATLAS was ejected and became an interstellar object,” said Salazar Manzano and his colleagues.
Taken together, these results underline that planetary systems and their celestial bodies can form under very different conditions. “This is evidence that the circumstances through which our solar system came into being do not have to be standard in the cosmos,” emphasizes co-author Teresa Paneque-Carreño from the University of Michigan. “That may sound obvious, but that too has to be checked and proven for the first time.” However, it is still unclear where exactly the interstellar comet 3I/ATLAS came from and which star it formed around.
Source: Luis Salazar Manzano (University of Michigan, Ann Arbor) et al., Nature Astronomy, 2026; doi: 10.1038/s41550-026-02850-5